Method of treating nitrated cellulose fibers



OCt. 15, W B. JONES METHOD OF TREATING NITRATED CELLULOSE FIBERS Filed Aug. 20, 1930 Jon/@6,

Patented Oct: 15, Y 1935 UNITED STATI-:s

METHOD F TBEATING CELLULOSE FIBERS W. Bartlett Jones, Chicago, lll., assignor; by mesne assignments, to Cellovis, Inc., Chicago,

Ill., a corporation of Illinois Application August 20, 1930, serial No. 476,561 l 1 Claim.

The present invention relates to the manufacture of derivatives of cellulose and like solids which are subjected to chemicals for reaction therewith to form derivatives having a solid form.

5 It has particular reference tobers of cellulose in the manufacture of esters of cellulose, such as the nitrateand the acetate.

The nitration of cotton or wood bers for the manufacture of nitro-derivatives, such as gun cotton, soluble cotton, and the lower nitrated forms exemplied by the collodiontype of materials, is chosen for the purpose cf illustrating the invention, its objects, its nature, and its advantages. It is chosen for the reason that large quantities of nitrocellulose are produced annually, and because its procedure is generally standardized and is perhaps oldest and best known of the esterifying processes.

It has long been a practice to treat cotton bers, ysuch as linters, and more recently treating chemical Wood fibers, with a nitrating acid known as mixed acid. This comprises a regulated mixture of sulphuric-acid and of vnitric acid and a regulated' proportion of water. The nitric acid is the essential lreactive ingredient, but the sulphurlc acid is present as a dehydrating agent to induce the reaction and to take up the water liberated in the reaction. Whether or not the sulphuric acid takes part in the reaction is immaterial.4 f l However, it is known that during the process there are formed some products of the ber constituents and sulphuric acid, which are not stable compounds. It is believed that other unstable compounds or products are also formed, perhaps of dehydration, which eventually become stable in the process regularly employed to treat the bers after the nitration reaction. This step or process is called stabilization, and it is to this .phase of the nitration process that thepreferred application of this invention relates. Other phases of the invention will also be discussed.

The mixed acid is a very strong and concentrated acid mixture, the unused portion of which is customarily recovered. The cellulose, Whether it be cotton bers, wood pulp bers, or other forms of cellulose, hereinafter referred to as pulp, is ordinarily separated from the acid by centrifugal force, the nitration mass of pulp and acid being dumped into Va. whirling wringen V'Ihis eiects ai partial separation` and large amounts of spent acid are readily recovered.

The pulp from the wringer contains residual acid which adheres vto it against the forces of th wringing, and other acid which is united to the pulp by bonds of a chemical nature. The pulp is customarily removed from the wringer and submerged in water, a. process designated as drowning. A large amount of residual acid that is held mechanically is removed by this 5 process 0f dilution. However, the drowning is not eiective to remove all the acid and other impuriv ties that must be removed or destroyed to produce stabilized nitrocellulose. Further treatment 'is carried out over a prolonged period of time 10 which.appears necessary to obtain the stabilized product of commerce. A number of variations in i subsequent treatment are known, and some are rigidly specied by ordinance divisions of the principal nations of the World for the production 154 of munitions. v

The U. S. Navy specifications are referred to in Technology of Cellulose Esters by Worden, Edition of 1921, Vol. 1, part IlI, pages 2260 to 2262. it is a very complex procedure and includes boilA 20 ing for 49 hours with not less than four changes of water. It is then pulped in fresh water in the presence of sodium carbonate. It is then poached by boiling 6 hours in fresh water, using carbonate of soda in specied proportion. Then it is boiled 25 for 6 hours again in clean water, with five changes of water, and finally is given ten cold water washings of one half hour each.` Further treatment is specified, all requiring manipulation and agitation. The specications include less 30 than 0.4% ash as a requirement.

The British specifications for stabilization, also found in the above reference, require a minimumtime of 42 hours with active manipulation. They require less than 2% mineral content in the dry 35 nitrocellulose.

All the procedures require heat,`agitation, ltratlon, chemical treatment.' large amounts of water, a long period of time, and a source of sufliciently pure water. In the present invention 40 the use of chemicals is dispensed with. A very small amount of water is required. Even water containing considerable amounts of mineral matter may be used, and the water itself becomes purified along with the nitrocellulose. Little or 45 no agitation is required, and manipulation may be minimized. Boiling is not necessary and no steam is required.

In all the common treatments for stabilization more or less mineral matter is present through the 50 natural hardnessof water or the materials used 'to treat the water, or because of the use of chemicals in the stabilizing process. The pulp, in the form of bers especialhr, has a very large surface area in proportion toits mass, and the electrical 55 forces of absorption cause considerable amounts of mineral and other impurities to be held to the surface of the pulp in such a way that ordinary processes of leaching or dissolution, as by water, do not completely remove these impurities, even though they may by nature be soluble. In consequence, the washed product now in general use contains adsorbed impurities.

One of the objects of the present invention is to stabilize nitrated cellulose pulp by subjecting it to the action of an electric current.

Another object of the invention is to treat pulp of the nature of nitrated cellulose to electrodialysis to remove both electropositive and electronegative impurities therefrom.

Another object of the invention is to break down by the use of an electric current the unstable compounds, especially those containing acid in their composition and which decompose or hydrolyze to liberate free acid in the usual stabilization processes.

A further object of the invention is to produce a solid acid derivative of a carbohydrate typeof solid, such as an ester of cellulose, like nitrocellulose which contains less adsorbed impurities than similar products treated by processes heretofore employed; which products may be made substantially free from acids and/or inorganic impurities, and colloidal impurities.

Various other and ancillary objects and advantages of the invention will become obvious or will hereinafter be referred to throughout the following description and explanation of the invention.

I have made tests upon commercial stabilized forms of soluble cotton containing, as purchased on the market, 30% ethyl alcohol, in order to determine whether the union between the nitric acid and the cellulose molecule is suiiciently stable to prevent destruction of the nitrocellulose when the latter is submerged in ordinary water and the mass subjected to the action of a direct current carried by the suspension. A sample of the commercial nitrocellulose was washed free of its alcohol content and submerged in water to form a thick slurry. y It was placed in a compartment cell and a current of electricity passed through it at a density of about 2 amperes per sq. ft. of cross sectional area 4of suspension for 24 hours, the temperature being at no time over '70 drowned product submerged in water and subjected to the passage of a direct current of electricity. A considerable amount of free and-loosely combined acid is present in drowned pulp, and it was found that the presence of these acids did not cause the ultimately vdesired nitrocellulose base to break down under the action of the cur.

rent may be suiciently stabilized for the formation of soluble cotton for use in lacquer.

As a speciiicexample of the invention the following detailed description and discussion is given, referring to the accompanying drawing as 5 an example of the type of cell which may be employed.

An insulating box III, such as a Wooden box, is divided into electrode and pulp compartments by suitable diaphragm material. A pulp chamber I I is formed having two opposing walls of diaphragm sheets, such as thick canvas or duck I2, capable of confining the pulp in a limited region of the liquid which lies in the cell on both sides of the two diaphragms, l

A cathode chamber I3 lies on one side of one diaphragm I2 and contains a suitable electrode I4 to function as cathode. It may be in the form of rods, slabs, or screen of nickel, Monel or other metal. .0

An anode compartment I5 lies behind the other diaphragm I2 and contains an anode I6, preferably of carbon or graphite to minimize corrosion. I prefer to have the anode compartment subdivided into an electrode compartment II and a flushing compartment I8 by an acid resisting diaphragm of low diffusing power, such as a porous silicaplate I9.

The compartments I3, I1 and I 8 have overow outlets 20, 2l and 22 respectively, and valved 30 water-feeding means 23, 24 and 25 respectively. Pulp compartment has valved outlet 26 at the the bottom with an insulating screen, such as cloth netting 21. A water-feeding means 28 is also indicated for adding water to the pulp com- 35 partment. y

Into the pulp chamber I place water in which may be suspended the solid product to be treated, such as the nitrated bers of chemical wood pulp derived from the drowning process which follows 40 nitration. The pulp suspension which isformed is indicated at 29, and may vary in concentration.

It ffhould not be-too thin, as the process will not operate efficiently. It should not be so thick that the fibers are tightly packed so as to substantially 45 close water channels through the mass. A thick slurry of about 5% nitrocellulose is satisfactory.

Water is placed in the remaining compartments, but ac id could be placed in the anode compartment containing the electrode in order fur- 50 ther to strengthen the acid. The level of the liquids in the various compartments is made such that there is a minimum hydrostatic pressure tov induce undesirable diiusion of liquid through the diaphragms. An electric potential is applied to 55 the electrodes preferably such that a direct current ows from anode to cathode, passing through the suspension. The voltage applied and the resistance of the cell will determine the strength of current which flows. A current of about 2 am- 60 peres per square foot of cross sectional area of suspension is suicient, but the exact value of the current is immaterial and affects primarily the time of the process. The greater the distance between the diaphragms, the longer will be the time 65 to bring the mass to a definite stage of completion for a given current. Increased current gives more heating effect, and the heat may be regulated by` control of the current or the application of cooling means if desired. ATemperature does not 70 materially affect the process.

I prefer to let the current flow for a suflicient length of time determined by the size of the cell, the strength of current. and the desired degree of completion. It may be continued until tests show the pulp to be suiiciently pure or sumciently stabiliz'ed, using any one of well known tests.

. For producing soluble cotton I prefer to use as raw material the pulp obtained by using the nitralow viscosity characteristics, commonly required for lacquers.l The product of this nitration process may be stabilized and used \for lacquer manufacture without the special treatments ordinarily employed for the lowering of viscosity characteristics. Accordingly the present invention used in combination with the said nitration process of said application has the advantage that the puried low-ash product obtained by electrodialysis need not be associated with water or other chemicals` to effect the lowering of viscosity, and there is therefore no opportunity in such a process for introducing additional impurities, such as inorganic matter and electrolytes.

Where requirements other than stability are made the current may be continued until all or a predetermined quantity of an impurity has been removed from the pulp, whether it be acid which moves to the anode compartment, or positive or negative colloidal particles which move toward the cathode or anode respectively, or metallic substances such as the ions of sodium, calcium, magnesium, etc., that move to the cathode.

A-MI prefer to flush the electrode compartments with fresh water, either continuously or intermittently to carry the migrated impurities out of the cell. In flushing the anode compartment I may consider the conservation or concentration of acid in the electrode compartment il, and may flush the compartment I8 when acid concentration therein is too highand likely to diffuse back s into the pulp chamber. By nthe arrangement acid will be heaviest and a considerable quantity t will be concentrated about the anode in the abscence of ushing. Later, near the end of the purification treatment it is desirable to have a low acid concentration on the anode side of the pulp chamber.

` It is to be considered that the present invention contemplates treatment of nitrated bers to stabilize them, of stabilizednitrocellulose and other solids to purify them, and of such other products as have been or may be hereinafter referred to.

The invention is particularly applicable to releasing adsorbed impurities not readily removed by dissolution processes, even though the impurities be ordinarily soluble; Adsorption is believed to be an electrical phenomena accompanied by a neutralization of opposite charges and the release may be accomplished in the cell described by opening the drain ze. The water then carries impurities during stabilization.

away the impurities which would, but for the cur'' rent, normally be adsorbed by the pulp. The separation may also, and would preferably Abe' effected by separating the pulp from the'liquid in such a way that when separated, the pulpisy out il; of the path of the current. Forlnitrocellulosethis will decrease the possibility of accident. An alternative procedure is to flush out the pulp 'l chamber with fresh water as current ilowsf through the mass, especially during the rst'partlgfj of the treatment at which time the acid concen# l tration is highest. Flushing may be intermittent or continuous.

The unstable compounds which are broken down in ordinary stabilization processes are lue-g1g?. lieved to 'include loose compounds betweenacid" and cellulose, such as sulphate esters. They are formed in concentrated acid, probably by the laws of mass action, and hence decompose in: diluter acids by the same laws. 'Ihe decompos'i'- 2dr-S tion, or hydrolysis, is slow and at best requires from 6 to 12 hours boiling, in acid, followed by manipulation to removeacid from the fiber. 'I'he present process of electrodialysis appears to speed up the. normal tendency of these compounds to 25 decompose, and completes the hydrolysis simultaneously with the removal of the acid. The effect of the current appears to be to remove the hydro-4 lyzed acid product from the source of hydrolyzing unstable compound, and also to weaken the forces 30 of union by reason of the electric potential. In accordance with these views the process of wash.- ing fibers during the ilow of current therethrough would be expected to stabilize the mass at an increased rate, as well as to purify the bers of adsorbed materials.

In the ordinary stabilizing processes where large amounts of water. are used and wasted, the pulp readily adsorbs minerals and other impurities and these may be carried away by the'pulp into the 40 remainder -of the steps employed upon the unaterial. These impurities account for the as h content of commercial nitrocellulose. They are present in finishing lacquers, and in gun cotton, and perhaps are one cause of the measurable deteri 4,5 oration of these materials, especially in light and heat. From other experiences with vthe eiIect of adsorbed mineral and other ingredients in fibrous material, I am of the impression that the impurities may catalyze destructive reactions. 50

'Ihe colloidal impuritiesf that can be removed by the process may be colored ones, and the reduction of them in quantity, or their absence, renders the product less colored,`and permits the manufacture of a better grade of product. Such 55 a product may be employed in lacquer formation to produce a lacquer, or` a nitrocellulose solution with a greater degree of clarity.

I do not limit the invention to removing the 'Ihe invention |50 in its'broadest aspect contemplates treating cellulose esters, or nitrocellulose which has been stabilized. to the action of an electric current in water suspension. Nitrocellulose or other esters treated or purified by means of this invention may have a reduced ash content, or maybe substantially ash-free. Nitrated cellulose may readn ily be obtained which'has less than 0.1% ash.

Purified esters may be employed with suitably pure organic solvents to preserve the favorable conditions secured by purifying the ester according to this invention. Solvents and other matenais to be associated with the purified product may be chosen 'with a view to the introduction of impurities. info a composition involving the purified ester. -Sumciently pure organic solvents may be obtained by distillation. Other organic substances may be employed in their best known pure forms, and purer forms 5 may readily be produced in some instances by well known'methods. -Gums and resins, plasticizers, and the like may be chosen which are f ree from mineral ingredients where it is desired to make low ash, or an ashf-freevproduct involving the puriied ester. Gun cotton rfor high explosives, pyroxylin plastics, laments and films, lacquers, and the like may be made which are highly pure and better than is permitted by use of products known heretofore. As an example of a composition involvingfa purified ester, thel following formula for a nitrocellulose lacquer is given, it being understood that i all the ingredients named are to be low in one or ""gmore'specic impurities, for example being ashfr`ee=\ Such a lacquer is useful in electrical insulation work, wherein the absence of acidity, electrolytes, ,.nninerala etc., gives it more desirable electrical properties. It is of course not essential in mak- -ing an improved iacquer or other composition that all the ingredients of the composition be.

highly purified, such as being ash-free. The use of ash-free or low-ash nitrocellulose of this invention contributes to a lower total ash for a given composition over that which would obtain using the present available forms of nitrocellulose.

In the case of lacquer the absence or the reduction in quantity of mineral and other impurities in the solid nitrocellulose gives to solutions of the nitrocellulose a greater degree of clarity' and freedom from color.

It is to be understood that much of the foregoing discussion refers to both cellulose acetate and cellulose nitrate. The invention contemplates processes and products of either variety of ester of cellulose, and of other materials in whichl like conditions exist, it being well known that other esters are known. I also contemplate application of the invention to other materials such as deriv# atives of cellulose which may be esteried without the base losing its solid form. Other forms of carbohydrates may be esteried in processes 5^* chemically analogous to those dealing with cellulose and its derivatives. For example, starch in granular form may be nitrated and acetylated without loss of its granular form. Such esteriied granules may be treated by eiectrodialysis with- 10 out departure from the spirit and scope of the present invention.

However, I' prefer to consider that the inven tion in its most practical embodiment is best applicable to fibrous materials, such as cellulose., 15 Fibers are relatively large as compared to granules; they mat well; they permit iiltrations without difficulty; they permit use of rather coarse and open diaphragms in the electric cell through whichcolloidal materials may easily migrate in 20 the process of purication. In certain respects fibers oiier special advantages in the conduction of electricity.

I consider the nitration of cellulose as the preferred field for practice of the invention. The 25 process `uses verylittle water and permits manufacturing operations where large amounts of wa.- ter are not economicallyv available. The water employed need not be pure and may be purified :fn the practice of the invention as an incident 30 i thereto. The acid removed may be concentrated and not lost as heretofore. 'I'he process is more Y.; rapid, and requires less equipment, labor and ma- "i nipulation. Steam and heat are not essential.

The product is distinctly purerv in itself than 35 products obtained heretofore, and it may be employed with associated substances to make higher grade commercial products than have heretofore been available.

I aim to set forth the invention in all the as 4.07. pects herein described and referred to, and in the .fm foregoing I do not limit the invention short of 4 its scope as presented in the accompanying claim.

Y I claim: i

A method of treating nitrated cellulose fibers derived from a nitration reaction which consists of suspending the bers in water, passing a current of electricity through the suspension, and changing the water during the flow of electric 50 current.

W. BARTLE'II JONES. 

